Erica P. John Fund, Inc. (EPJ Fund) (plaintiff) is an investor in Halliburton Co. (Halliburton) (defendant). EPJ Fund brought this suit on behalf of all purchasers of Halliburton common stock between June 3, 1999, and December 7, 2001, alleging that Halliburton made misrepresentations in order to inflate its stock price in violation of § 10(b) of the Securities and Exchange Act of 1934. EPJ Fund sought class certification under Federal Rule of Civil Procedure 23. The District Court found that the plaintiffs could not obtain class certification because they were unable to prove loss causation. The Court of Appeals affirmed. This Court granted certiorari.